Avalanche Photodiode Detector

ABSTRACT

An avalanche photodiode detector is provided. The avalanche photodiode detector comprises an absorber region having an absorption layer for receiving incident photons and generating charged carriers; and a multiplier region having a multiplication layer; wherein the multiplier region is on a mesa structure separate from the absorber region and is coupled to the absorber region by a bridge for transferring charged carriers between the absorber region and multiplier region.

1. FIELD OF THE INVENTION

The present invention relates generally to photodiode detectors, andmore particularly, to avalanche photodiode detectors.

2. BACKGROUND

Avalanche photodiode detectors (APDs) are photosensitive devices thatdetect optical power by converting an input signal (photons) to anelectrical signal. The input signal is amplified by an “avalancheeffect” when carriers are injected in an area with high electricalfield. This occurs because multiple electron-hole pairs are created foreach absorbed photon.

An APD typically comprises of a plurality of stacked layers including amultiplication layer and an absorption layer on a semiconductorsubstrate. The absorption layer absorbs incident photons to createelectron/holes that are transferred to the multiplication layer. Themultiplication layer multiplies the electrons/holes. This occurs whenelectron/holes have sufficient energy to create a new electron and hole.Initial carriers and newly created carriers may create additionalelectrons and holes (hence the name “avalanche”) by repeating themultiplication process.

In a conventional APD, all layers are grown in one epitaxial growth.This may lead to some interface defects. Due to interface defects theremay be some carrier traps and recombination centers, which reduceoverall quantum efficiency and after pulsing performance of an APD.

Furthermore, simultaneous growth absorption and multiplication layersdoes not provide flexibility in selecting different materials for theselayers.

Therefore there is a need for an avalanche photodiode that overcomes theforegoing problems in conventional APDs.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an avalanche photodiode detectoris provided. The avalanche photodiode detector comprises an absorberregion having an absorption layer for receiving incident photons andgenerating charged carriers; and a multiplier region having amultiplication layer; wherein the multiplier region is on a mesastructure separate from the absorber region and is coupled to theabsorber region by a bridge for transferring charged carriers from theabsorber region to the multiplier region.

In another aspect of the present invention, an avalanche photodiodedetector is provided. The avalanche photodiode detector comprises anabsorption layer formed over a semiconductor substrate, for receivingincident photons and generating charged carriers, a contact layer formedon the absorption layer; a first P-doped layer formed on the absorptionlayer; a multiplication layer formed over the contact layer; a second Pdoped layer formed on the multiplication layer; a first contact providedon the second P-doped layer; a second contact provided on the contactlayer; and a third contact provided on the substrate; wherein thevoltage difference across the first contact and the second contactcontrols the multiplication layer, and the voltage difference betweenthe second contact and the third contact controls the absorption layer.

In yet another aspect of the present invention, an avalanche photodiodedetector is provided. The avalanche photodiode detector comprises anabsorber region having an absorption layer for receiving incidentphotons and generating charged carriers; a multiplier region having amultiplication layer; wherein the multiplier region is built on aseparate mesa structure, and may be of a different material than theabsorber region; and is wired to the absorber region to transfer chargedcarriers.

This brief summary has been provided so that the nature of the inventionmay be understood quickly. A more complete understanding of theinvention may be obtained by reference to the following detaileddescription of the preferred embodiments thereof in connection with theattached drawings.

BRIEF DESCRIPTION CF THE DRAWINGS

The foregoing features and other features of the present invention willnow be described with reference to the drawings of a preferredembodiment. In the drawings, the same components have the same referencenumerals. The illustrated embodiment is intended to illustrate, but notto limit the invention. The drawings include the following Figures:

FIG. 1 shows a block diagram of an APD structure;

FIG. 2A shows a schematic of an APD structure on separate mesas,according to an aspect of the present invention;

FIG. 2B shows the top view of the APD structure of FIG. 2A;

FIG. 2C shows a schematic of a APD structure with a N-Contact andquantum wells, according to an aspect of the present invention;

FIG. 3 shows an APD structure with single mesa, according to an aspectof the present invention; and

FIG. 4 shows a schematic of a hybrid APD structure, according to anaspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one aspect of the present invention, a three terminal APD structurewith separate absorption and multiplication layer (also referred to as“TT-SAM APD or APD”) is provided. The absorption layer andmultiplication layer may be grown separately and hence are controlledindependently. This allows one to select different materials for theabsorption and multiplication layer. The APD of the present inventionalso provides an additional terminal. The additional terminal enablesindividual control of bias across an absorption (Absorber”) region and amultiplication (“Multiplier”) region.

To facilitate an understanding of APD structure, a general overview of aconventional APD structure will be described. The specific structuralcomponents and layers of APD of the present invention, will then bedescribed with reference to general structure of APD.

FIG. 1 shows a top level block diagram of a conventional APD structure.APD 100 includes a P—InP substrate layer 110; a P—InP buffer layer 102and an N—InP multiplication layer 103; an n-InGaAsP grading (orbandgap-transition) layer and charge layer 104 of an intermediatebandgap; and an n-InGaAs narrow-bandgap absorption layer 105. Chargelayer 104 is generally provided to control the electric fields in theabsorption and multiplication layers, 103 and 105, respectively, andreduce charge accumulation at the interface between them. Layers 106 and107 are metal contacts, which may be made of, for example, AuInZn orAuSn.

During operation of APD 100, incident photons 101 are absorbed inabsorption layer 105, and charged carriers (holes and electrons) arecreated through a photogeneration process. Charged carriers aremultiplied in multiplication layer 103 resulting in internal gain withinAPD 100. Incident photons 101 may enter APD through substrate layer 110(as shown in FIG. 1) or through absorption layer 105.

In APD 100, electric field profile is controlled by charge layer 104. Toensure a low electric field in absorption layer 105 and a high field inmultiplication layer 103, doping level in charge layer 104 needs to beprecisely controlled. In practice, it is difficult to control dopinglevel in charge layer 105. The present invention provides an APDstructure that eliminates use of charge layer, overcoming inherentproblems associates with the use of charge layer.

FIGS. 2A and 2B show an APD structure according to one aspect of thepresent invention. FIG. 2A is a cross-sectional view and FIG. 2B is atop view of APD structure 200.

Referring to FIG. 2A, a three terminal SAM APD structure 200A (alsoreferred to as “TT-SAM” or “APD” interchangeably) is shown. APD 200Acomprises of separate absorber region 203A and multiplier region 211Aformed over a semiconductor substrate layer 208. Substrate layer 208 ispreferably a semi-insulating layer. In one aspect, substrate layer 208may be an indium phosphide (InP) layer. Absorber region 203A andmultiplier region 211A may be grown in one epitaxial run or separately.

Absorber region 203A includes a first insulating layer 207A. Insulatinglayer 207A is formed of InAlAs and may have a thickness between 0.7μm-1.5 μm. Above first insulating layer 207A, a second insulating layer207 is formed. Second insulating layer 207 is formed of InAlAs and has athickness of between 0.2 μm-1.5 μm. Above second insulating layer 207, athird insulating layer 205 is provided. This third insulating layer 205may be formed of InP and has a thickness of about 0.2 μm to 0.5 μm.

Above third insulator layer 205, an N-doped contact layer 204 is formed.N-doped contact layer 204 is formed of a suitable material havingappropriate thickness. In one aspect, N-doped contact layer 204 isformed of InP, and may have a thickness between 0.5 μm-1.0 μm.

First insulating layer 207A, second insulating layer 207 and thirdinsulating layers 205 act as insulators and do not allow flow ofcharge/electrons to the substrate layer 208. This also prevents anyunintentional leakage of current through N-doped contact layer 204 tothe substrate layer 208.

Above contact layer 204, an absorption layer 203 is provided. Absorptionlayer 203 may be formed of a material having bandgap of 0.5-0.7 eV. Inone aspect, absorption layer 203 is formed of InGaAs, and may have athickness between 1-5 μm.

Above absorption layer 203, a wide bandgap window layer 201 may beprovided. In one aspect, window layer 201 may be formed of InP orInGaAsP. Contacts 202 and 206 are also provided on absorber region 203A.A separate bias is applied across absorber region 203A and potentialdifference between contacts 202 and 206 controls absorber region 203Afunctionality.

Multiplier region 211A includes an N-doped layer 207B. N-doped layer207B is preferably formed of InAlAs and has a thickness of between of0.7 μm to 1.5 μm.

A multiplication layer 211 is grown on N-doped layer 207B. In oneaspect, multiplication layer 211 comprises InAlAs. Multiplication layer211 may have a thickness range of 0.02 μm to 1.5 μm.

It is within the scope of the present invention to use other suitablematerials known in the art to form absorption layer 203, contact layer204, window layer 201, multiplication layer 211, N-doped 207B, P-dopedlayer 210 or substrate layer 208.

Above multiplication layer 211, a P-doped InP layer 210 is grown.P-doped layer 210 may have a thickness in the range of 0.2 μm to 0.5 μm.

Contacts 209 and 212 are also provided in multiplier region 211A.Contact 212 is provided over a conducting layer 212A. Conducting layer212A is formed over the substrate layer 208. Conducting layer 212A has athickness of 0.35 μm to 0.75 μm.

A separate bias is applied across multiplier region 211A and voltagedifference between contacts 209 and 212 controls the electric field inmultiplication layer 211.

Absorber region 203A and multiplier region 211A are joined by a metalcontact bridge 206A via contacts 206 and 206B. Metal bridge 206Atransfers charge between absorber region 203A and multiplier region211A. A passivation layer 206C (an insulator region) may also beprovided beneath the metal bridge 206A.

The bias condition of Schottky junction 206D, which is influenced byvoltage difference between contacts 202 and 209, controls carrierinjection from absorber region 203A to multiplier region 211A. 206 and206B are metal contacts and may be N-metal contacts, while contacts 202and 209 may be P-metal contacts. Metal bridge 206A and metal contacts206 and 206B may be a formed of a connected metal having the samepotential.

Incident photons 220 enter APD structure 200A via layer 201. Incidentphotons 220 are absorbed in absorber region 203A, and charged carriers(holes and electrons) are created through a photogeneration process. Thecharged carriers are injected into multiplier 211A and initiate anavalanche multiplication resulting in internal gain within APD 200A.

Absorber region 203A and multiplier region 211A are decoupled and bothregions have individual isolated mesas. Therefore the size and type ofmaterial for absorber region 203A and multiplier region 211A can becontrolled independently.

For APD 200A, dark current, dark count rate (DCR), device capacitanceand bandwidth benefit from a smaller multiplier region 211A area. Alarger absorber region 203A area with low electric field collectsphotons efficiently without significant sacrifice in all these aspects.

To maintain efficient carrier injection from absorber region 203A intomultiplier region 211A, a Schottky junction 206D is used in contact withundoped multiplication layer 211. Schottky junction 206D is used toinject photon-generated electrons into multiplier region 211A. Thisjunction is adjacent to but not in the primary carrier path in theavalanche process and hence avoids recombination by holes generated byavalanching in the multiplier region 211A. The avalanche-generated holesare collected at contact 209.

APD 200A does not have a charge layer. Instead of charge layer, thecarrier injection from absorber region 203A to multiplier region 211A iscontrolled by the bias condition of Schottky junction 206D, which isinfluenced by the 2-D potential distribution at the injection contact.In operation, this distribution is determined by the voltage differencesbetween contacts 202 and 209, and 209 and 212.

In an alternative embodiment shown in FIG. 2C, injection efficiency ofan APD 200B may further be improved with an N-well (210B) and quantumwells (210A). By replacing Schottky junction 206D with a p-n junction,leakage current from injection is minimized 2-D electron gas formed inquantum well (210A) transports injected electrons to a high field regionquickly and effectively while keeping holes from deviating from themultiplier region 211A. In the photon counting applications, becausethere is no hole to recombine in the beginning of a gate pulse, theinjected elections will trigger the avalanche events effectively, orrealize higher quantum efficiency. In one aspect of the presentinvention, quantum well 110A may have a thickness range from 50 Å-100 Å,and the N-well may be formed by diffusion or ion-implantation.

It is within the scope of present invention to simultaneously grow apart of absorber region and multiplier region. This simultaneously grownregion may then be separated and processed independently. As shown inAPD 200A and 200C, first insulating layer 207A and N-doped layer 207Bare formed of InAlAs. After growing a layer of InAlAs, it is separatedinto at least two parts. One part, 207A, forms an insulating layer whilethe other part is doped to form N-doped layer 207B. Similarly,multiplication layer 211 and P-doped layer are grown simultaneously assecond insulating layer 207 and third insulating layer 205.

FIG. 3 shows APD 300 in another aspect of the present invention. APD 300includes a single mesa structure. Substrate layer 307 is formed of asemiconductor material, for example InP. Substrate layer 307 may have athickness range of 200 μm-500 μm. An absorption layer 306 of thickness 1μm-5 μm is grown on substrate layer 307. Absorption layer 306 ispreferably an InGaAs layer. Above absorption layer 306, a P-doped layer305A is formed. P-doped layer 305A may have a thickness of 1 μm to 5 μm.

A N-doped layer 305 of InP is formed over P-doped layer 305A. N-dopedlayer 305 may have a thickness of about 0.1 μm. A multiplication layer302 of InP or InAlAs is formed on doped layer 305. Multiplication layer302 may have a thickness between 0.02 μm-1.0 μm. A P-layer 301 of InP isformed above multiplication layer 302 having a thickness between 0.2μm-1.0 μm.

It is within the scope of the invention to use other suitable materialsknown in the art to form absorption layer 306, P-doped layer 305A,N-doped layer 305, multiplication layer 302 or P-layer 302.

Contact 310 is provided over P-layer 301 while N-doped layer 305 has twocontacts 303 and 304. Photons 309 enter via substrate layer 307.Incident photons 309 are absorbed in absorption layer 306, and chargedcarriers (holes and electrons) are created through a photogenerationprocess. The charged carriers initiate an avalanche multiplication inmultiplication layer 302 resulting in internal gain within APD 300.

In APD 300, charge is injected when electrons pass through N-doped layer305. In order to maintain efficient carrier injection and reverse biasin the absorption layer 306 while sustaining electric field uniformityin multiplication layer 302, a tunnel junction with a p-type layerbetween 305 and 306 is inserted between the two function regions.

APD 300 also does not have a charge layer. Instead of charge layer, biasdifference between contacts 310 and 303 controls multiplication layer302. Similarly, voltage difference between contacts 304 and 308 controlsabsorption layer 306.

APD configuration 300 has a larger absorption layer 306 area coupled toa smaller multiplication mesa 302. The smaller multiplication mesa 302reduces dark count rate while minimizing bias across absorption layer306, thus increasing overall usable quantum efficiency of APD 300.

In yet another aspect the present invention, a hybrid APD 400 as shownin FIG. 4, is provided. By hybrid it means that different materialshaving different characteristic properties may be used for formingabsorber region 400A and multiplier region 400B of APD 400.Conventionally, it has been difficult to integrate Silicon material withInGaAs or InP or similar materials to form APDs. In one aspect of thepresent invention, hybrid APD 400, overcomes these material constraints.

In APD 400, different materials for multiplier region 400B and absorberregion 400A are used, and wafer bonding may be used to join thedifferent materials.

Absorber region 400A is preferably formed of InP while multiplier region400B may be formed of Silicon. It is well known that material growthconstraints exist between materials like InGaAs or InP and silicon.However, with hybrid integration, these material growth constraintsbetween absorber and multiplier regions (400A and 400B) are avoidedproviding a wider spectrum of materials available for deviceoptimization.

Absorber region 400A includes a P-doped layer 415 of InP or InAlAs ofthickness range of 0.2 μm-1.0 μm. Absorption layer 413 of InGaAs isformed over P-doped layer 415. Absorption layer 413 may have a thicknessrange of 0.2 μm-0.5 μm.

An N-doped layer 412 of InP, InAlAs or InGaAsP is formed over absorptionlayer 413. N-doped layer 412 may have a thickness range of 0.2 μm-0.5μm. Contacts 411 and 414 control the bias across absorption layer.

Multiplier region 400B includes a silicon substrate layer 409. Aninsulator layer 408 of silicon oxide is formed over silicon substratelayer 409. Above insulator layer 408, there is N-well region (405) andP-well region (407). A multiplication layer 406 is provided betweenN-well region (405) and P-well region (406) for transferring high fieldfrom N-well to P-well.

Absorber region 400A is wire bound (403) to a multiplier region 400B viametal contacts 402 and 411. Schottky junction 402 is employed to improveinjection efficiency for photon-generated carriers. In order to avoidrecombination with avalanche-generated holes, Schottky junction 402avoids mainstream avalanche current, while a small potential differencebetween Vpin+ (404) and Vm− (401) helps to inject electrons.

Voltage difference between contacts 401 and 404 determines bias formultiplier region 400B while voltage difference between contacts 411 and414 determines bias for absorber region 400A.

Incident photons 410 enter absorber region 400A through N-doped layer412. Incident photons 410 are absorbed in the absorber region 400A, andcharged carriers (holes and electrons) are created through aphotogeneration process. The charged carriers are injected through theSchottky junction (402) and initiate an avalanche multiplication inmultiplier region 400B resulting in internal gain within APD 400.

APD 400 eliminates charge layer and interface defects associates withthe use of charge layer. APD 400 also allows one to select differenttypes of materials for multiplier region 400B and absorber region 400A

The foregoing APDs (200A, 200B, 300 and 400) the present inventionprovide better quantum efficiency. APDs of the present inventioneliminate use of charge layer between absorption layer andmultiplication layer by introducing an extra terminal between absorberregion (203A or 400A) and multiplier region (211A or 400B). The electricfields in absorber (203A or 400A) and multiplier regions (211A or 400B)are controlled individually by the potential differences between thethree terminals. The decoupling of the two functional regions,multiplier region and absorber region, helps in maintaining a low butsufficient field in the absorber. This also allows one to independentlyoptimize the size, material and layer structure of the two regions.

While the adaptive aspects of the present invention list specificmaterials with specific thickness for forming absorption layer,multiplication layer, contact layer, P-doped layer and N-doped layer, itwill be understood by those skilled in the art that similar materials,exhibiting similar properties of varying thickness may be used, andequivalents may be substituted for elements thereof without departingfrom the true scope of the adaptive aspects of the present invention.

Although the present invention has been described with reference tospecific embodiments, these embodiments are illustrative only and notlimiting. Many other applications and embodiments of the presentinvention will be apparent in light of this disclosure and the followingclaims.

1. An avalanche photodiode detector, comprising: an absorber regionhaving an absorption layer for receiving incident photons and generatingcharged carriers; and a multiplier region having a multiplication layer;wherein the multiplier region is on a mesa structure separate from theabsorber region and is coupled to the absorber region by a bridge, fortransferring charged carriers from the absorber region to the multiplierregion.
 2. The avalanche photodiode detector of claim 1, wherein theabsorber region further includes a pair of first contacts.
 3. Theavalanche photodiode detector of claim 2, wherein bias across theabsorber region is maintained by the first contacts.
 4. The avalanchephotodiode detector of claim 1, wherein the multiplier region furtherincludes a pair of second contacts.
 5. The avalanche photodiode detectorof claim 4, wherein bias across the multiplier region is maintained bythe second contacts.
 6. The avalanche photodiode detector of claim 1,wherein the absorber region and the multiplier region are formed over asemiconductor substrate layer.
 7. The avalanche photodiode detector ofclaim 1, wherein a Schottky junction is used for injecting carriers inthe multiplication layer.
 8. The avalanche photodiode detector of claim1, wherein a quantum well and an N-well is used for injecting carriersin the multiplication layer.
 9. The avalanche photodiode detector ofclaim 1, wherein the bridge is a metal bridge.
 10. The avalanchephotodiode detector of claim 1, wherein the absorption layer has alarger surface area then the multiplication layer.
 11. An avalanchephotodiode detector, comprising: an absorption layer formed over asemiconductor substrate, for receiving incident photons and generatingcharged carriers; a contact layer formed on the absorption layer; afirst P-doped layer formed on the absorption layer; a multiplicationlayer formed over the contact layer; a second P doped layer formed onthe multiplication layer; a first contact provided on the second P-dopedlayer; a second contact provided on the contact layer; and a thirdcontact provided on the substrate; wherein the voltage difference acrossthe first contact and the second contact controls the multiplicationlayer, and the voltage difference between the second contact and thethird contact controls the absorption layer.
 12. The avalanchephotodiode detector of claim 11, wherein the absorption layer has alarger area then the multiplier layer.
 13. An avalanche photodiodedetector, comprising: an absorber region having an absorption layer forreceiving incident photons and generating charged carriers; a multiplierregion having a multiplication layer; wherein the multiplier region isbuilt on a separate mesa structure and is of a different material thenthe absorber region; and is wired to the absorber region to transfercharged carriers.
 14. The avalanche photodiode detector of claim 13,wherein the absorber region includes a pair of first contacts.
 15. Theavalanche photodiode detector of claim 14, wherein bias across theabsorber region is maintained by the first contacts.
 16. The avalanchephotodiode detector of claim 13, wherein the multiplier region furtherincludes a pair of second contacts.
 17. The avalanche photodiodedetector of claim 16, wherein bias across the multiplier region ismaintained by the second contacts.
 18. The avalanche photodiode detectorof claim 13, wherein a Schottky junction is used for injecting carriersin the multiplication layer.